Refactorings for AbstractAlexaApplication, TextHelper and IntentManager

[RalfEggert]

The AbstractAlexaApplication needs to be refactored:

• [x] CertificateValidator should be removed from AbstractAlexaApplication
• [x] Check of application id should be removed from AbstractAlexaApplication
• [x] Both checks should be added to the middleware pipeline
• [x] TextHelper should be removed from AbstractAlexaApplication
• [x] Locale for TextHelper should be set in the middleware pipeline
• [x] TextHelper should be injected in the intent classes via factory

Further refactorings:

• [x] Remove the need to extends the text helper class for each skill
• [x] Remove the need to extends the intent manager class for each skill
• [x] Add a new configuration class for current skill, e.g. for application id, intents, etc.

[RalfEggert]

Closed by #21